1. Field of the Invention
The present invention relates to a superhard film-coated member for use in cutting or wear resistant tools, wear resistant parts and components, optical parts and components, electronic materials and the like, which is produced by forming a film of diamond and/or diamond-like carbon on the surface of a cemented carbide substrate by a chemical vapor deposition process. The term "superhard film" herein referred to shall generally mean films formed of diamond and/or diamond-like carbon.
2. Description of the Related Art
Chemical vapor deposition ("CVD"), in providing diamond coating on the surface of a cemented carbide substrate, tends to produce amorphous carbon on the cobalt in the bonding phase (hereinafter also referred to as "bonding cobalt", and such bonding cobalt and like metals in the bonding phase as "bonding metals"), which hinders formation of diamond. As one solution to this problem, the prior art, exemplified by Japanese Patent Publication No. Sho 63-20911, discloses that acid etching the substrate to remove the bonding metal to a predetermined depth from its surface has the effect of allowing formation of the diamond film thereon and remarkably improving the bonding strength between the diamond film and the substrate.
Further, Japanese Patent Publication Laid-Open No. Sho 63-100182 discloses that a cemented carbide of which the content of bonding cobalt is reduced by about 1 to 4% by weight is suitably used as a substrate for diamond coating, but that for diamond film formation such a low-cobalt cemented carbide still requires acid etching to have its bonding cobalt removed.
Alternatively, Japanese Patent Publication Laid-Open No. Sho 62-67174 discloses several methods of bonding metal removal, including dry etching the substrate in a plasma of carbon fluoride, sputter etching with hydrogen, argon gas, etc., in addition the aforementioned method of using an acid.
There has been proposed a formation of an intermediate layer to improve the bonding strength between the diamond film and the substrate without removing the bonding phase as described above. For example, Japanese Patent Publication Laid-Open No. Sho 63-1280 discloses forming on the surface of a cemented carbide or a like substrate a layer of a carbide, nitride, boride, etc. of an element of Group IVa, Va or VIa of the periodic table, or a compound or a mixture of these, and then providing a diamond film on the thus formed layer.
Also, as to the thickness of the superhard film coating, Japanese Patent Publication Laid-Open No. Hei 1-212491 describes that a thickness exceeding 20 micrometers is not desirable because the thicker film coating will delaminate due to thermal stresses occurring in the coating.
As described hereinabove, many proposals have been made for improving the bonding strength of the diamond film to the substrate, however no suitable methods which assure satisfiable quality and are adapted for industrial production have been found yet. Accordingly, a need still exists for such a suitable method.
In other words, since a cemented carbide subjected to acid etching has the bonding metal thereof removed from its surface layer, WC (tungsten carbide) particles existing in the interface between the substrate and the diamond film are not always firmly retained at their fixed positions (in a case using WC as the substrate). A diamond film formed on such an interface cannot have a sufficient bonding strength, and thus the resultant cutting tools are limited in their usage to cutting of low (below 12%) silicon-aluminium alloys, graphite, carbon fiber-reinforced plastics, green ceramics, etc. That is, such cutting tools are inappropriate for interrupted cutting of 18-20% Si--Al alloys, heavy-duty cutting such as high feed cutting, and deep cutting over a prolonged period of time.
In addition, if a diamond film grows to a thickness close to 15 micrometers on an acid etched cemented carbide, it will delaminate. Thus, the prior art process provides for only a diamond film having a thickness in the range of about 0.1 to 5 micrometers because of concerns over such delamination; consequently the resultant tools will have their substrates exposed before their flank wears (Vb), used as a measure of tool life, reach a predetermined amount, even if delamination of the diamond film could be avoided. Thus, the resultant tools will inevitably have a limited useful life.
Accordingly, the present invention solves the foregoing problems of the prior art by providing an improved cemented carbide coated with a superhard film having a high bonding strength, the film being allowed to grow thicker to produce a superhard film-coated cemented carbide with a longer life, and by providing an improved and novel method for producing such a diamond-coated cemented carbide.